Acid-base concepts HSAB theory

W. B. Jensen, The Lewis Acid-Base Concepts An Overview, Wiley-Interscience, New York, 1980, and H. L Finston and Allen C. Rychtman, A New View of Current Acid-Base Theories, John Wiley Sons, New York, 1982, provide good overviews of the history of acid-base theories and critical discussions of the different theories. R. G. Pearson s Hard and Soft Acids and Bases, Dowden, Hutchinson, Ross, Stroudsburg, PA, 1973, is a review by one of the leading exponents of HSAB. For other viewpoints, the references provided in this chapter should be consulted. 

These concepts play an important role in the Hard and Soft Acid and Base (HSAB) principle, which states that hard acids prefer to react with hard bases, and vice versa. By means of Koopmann s theorem (Section 3.4) the hardness is related to the HOMO-LUMO energy difference, i.e. a small gap indicates a soft molecule. From second-order perturbation theory it also follows that a small gap between occupied and unoccupied orbitals will give a large contribution to the polarizability (Section 10.6), i.e. softness is a measure of how easily the electron density can be distorted by external fields, for example those generated by another molecule. In terms of the perturbation equation (15.1), a hard-hard interaction is primarily charge controlled, while a soft-soft interaction is orbital controlled. Both FMO and HSAB theories may be considered as being limiting cases of chemical reactivity described by the Fukui ftinction. 

The HSAB theory of Pearson has been one of the key organizing concepts in the study of nucleophiles. This theory is applied and examined in Chapters 15 and 16. In Chapter 15, Fuji applies the HSAB principles to design nucleophilic reagents for cleaving C-X bonds. Fuji notes that all bonds are made of a combination of Lewis acid and Lewis base and have hard-soft dissymmetry for the typical C-X bond, the carbon is a soft acid and the X is a hard base. Thus, in accord with the HSAB principles, a soft base (the nucleophile) and a hard acid are required to cleave this bond selectively. Applying these ideas, Fuji then shows the utility of several soft base-hard acid reagents for cleaving various C-X bonds in complex molecules. 

Modern advances in solvation theory are reviewed.A second critical review of the thermodynamic functions and crystallographic data of some solid solvates (e.g. NaI,3MeOH LiCl,py and CoCl2,6NHg) shows that the relative acid-base properties of the constituent cation, anion, and solvate molecule can well be described by the HSAB (hard and soft acids and bases) concept. ... 

The really serious objection to the HSAB concept was that no exact definition of hardness existed. There was no defining statement based on existing theory, nor was there an operational definition linking hardness to experiment. As a result, it was not possible to give an experimental, or theoretical, value to t], for various acids and bases. The concept could not be quantified. 

In the 1960s, Ralph G. Pearson introduced hard and soft acid and base (HSAB) theory [10, 11], The HSAB concept is widely applied in chemistry for explaining characteristics of compounds, reaction mechanisms, and reaction pathways. In the initial studies, Lewis acids and Lewis bases were classified as hard or soft (Table 1). 

The concept of hard and soft acids and bases ( HSAB ) should also be mentioned here. This is not a new theory of acids and bases but represents a useful classification of Lewis acids and bases from the point of view of their reactivity, as introduced by R. G. Pearson. 

To the extent that the N+ correlation is successful it means that the pattern of nucleophilic reactivity is not influenced by the nature of the electrophilic center at which substitution takes place. On the other hand, according to the concepts of the theory of hard and soft acids and bases (HSAB) as applied to nucleophilic substitution reactions (Pearson and Songstad, 1967) one would expect that a significant change in the HSAB character of the electrophilic center as an acid should lead to changes in the pattern of nucleophilic reactivity observed. Specifically, in substitutions occurring at soft electrophilic centers, soft-base nucleophiles should be more reactive relative to other nucleophiles than they are in substitutions at harder electrophilic centers, and in substitutions at hard electrophilic centers hard-base nucleophiles should appear relatively more reactive compared to other nucleophiles than they do in substitutions at softer electrophilic centers. 

Later on, Pearson [75] introduced the concept of hard and soft acid and bases (HSABs) hard acids (defined as small-sized, highly positively charged, and not easily polarizable electron acceptor) prefer to associate with hard bases (i.e., substances that hold their electrons tightly as a consequence of large electronegativities, low polarizabilities, and difficnlty of oxidation of their donor atoms) and soft acids prefer to associate with soft bases, giving thermodynamically more stable complexes. According to this theory, the proton is a hard acid, whereas metal cations may have different hardnesses. 

---